1. Field of the Invention
The present invention relates generally to an ink jet printing apparatus. More specifically, the invention relates to an ink jet printing apparatus which can reduce a streak or a banding which can be formed at a boundary between two scanned regions in a recorded image.
2. Description of the Related Art
An ink jet printing apparatus is popularizing rapidly in the recent years for various advantages, e.g., low noise as a non-impact type printing apparatus, ease of printing a color image by employing a plurality colors of inks, and so forth.
FIG. 1 is a general perspective view of one example of a conventional ink jet printing apparatus.
In FIG. 1, a printing medium 5 wound in a form of a roll, is pinched between a pair of paper feed rollers 3 via a transporting rollers 1 and 2, and is fed in a direction f shown in the drawing by driving of an motor 50 a driving force of which is transmitted to the paper feed rollers 3. Across the printing medium 5, guide rails 6 and 7 are arranged in parallel relationship to each other. A printing head unit 9 mounted on a carriage 8 can scan transversely along the guide rails. The printing head unit 9 has heads 9Y, 9M, 9C and 9Bk respectively corresponding to four color inks of yellow, magenta, cyan and black and four ink tanks (not shown) corresponding to respective of four colors. The printing medium 5 is intermittently fed in a length corresponding to a width of ejection opening array of each head of the printing head unit 9. During an interval between feeding operations of the printing medium, the printing head unit 9 moves in a direction P shown in the drawing to eject ink droplets depending upon an image signal to perform printing.
In such serial scanning type ink jet printing apparatus, a density fluctuation of streak state may be caused at a boundary between portions respectively corresponding to respective scanning of the printing head to be a cause of degradation of an output image. This density fluctuation may be observed such as a band, and therefore, this density fluctuation is also called "banding". In discussion below, this visible phenomenon is referred as "streak."
As a printing apparatus solving such problem, there have been proposed an apparatus, in which an image signal corresponding to printing element located at the end among a plurality of printing elements, such as ink ejection openings of the printing head, is corrected for reducing a streak form density fluctuation at the boundary as set forth above. Another proposal is an apparatus in which of scanning is performed a plurality of times in respect of each pixel with mutually distinct plurality of printing elements, namely, multi-scanning printing is performed, to make a streak at the boundary not perceptible.
With the construction set forth above, it may provide some effect in reduction of the streak at the boundary. However, the effect of reduction of the streak at the boundary can be small for certain types of the printing medium. This is because the prior art as set forth above principally takes fluctuation of a feeding amount of the printing medium or bleeding of the ink in the printing medium, as a cause of the streak at the boundary portion. In connection with this, according to the invention's study of results of experiments, the streak can be caused by a different phenomenon. More specifically, it has been found that beading of the ink on the printing medium can also cause the streak at the boundary.
This is clearly different from the streak caused by fluctuation of feeding amount or so forth. For example, when the streak at the boundary is caused by fluctuation of the feeding amount, if the feeding amount is excessively large, a region not printed is remained to cause a white streak, and if the feeding amount is too small, a part of image in preceding and following scanning cycle overlaps to form a region having excessively high density to form a black streak. As a method for reducing such streak, there have been proposed methods to enhance precision of feeding amount as a first method, to set paper feeding amount to slightly overlap images in preceding scanning cycles and to thin the data at the overlapping portion, or to control ink ejection amount in the ink jet printing system as a second method. Even when the scanning images in preceding and following scanning cycles are precisely connected in the first method or when the data is thinned at the overlapping portion in the second method, beading can be caused at the boundary of each cycle of scanning image to cause the streak at the boundary.
More specifically, beading is a phenomenon that during a period from the timing of adhering of the ink droplet to the printing medium to completion of absorption, adjacent ink droplets are contacted to each other to cause admixing of the ink droplets due to surface tension of the ink.
Further discussion will be given hereinafter with respect to beading with reference to FIGS. 2 and 3.
FIG. 2 shows 128 in number of ink ejection openings in each of the heads shown in FIG. 1, and positional relationship of these ejection openings between respective heads. Namely, in respective heads, mutually corresponding ejection opens are located at the same position in the feeding direction of the printing medium (up and down direction on the drawing).
FIGS. 3A to 3D are illustrations showing a process to cause beading and influence thereof.
As shown in FIG. 3A, when a plurality of ink droplets are ejected at high density with respect to a printing width d, the ink droplets hitting the printing medium are in mutual contact before penetrating into the printing medium to cause beading between the ink droplets as shown in FIG. 3B.
Then, among an ink droplet group aligned in the printing width d, respective ink droplets of a central part of aligned droplet group contact with left and right adjacent ink droplets to be influenced to equalize forces to be applied to the ink droplets to become not motive. In contrast, the ink droplets at both ends of the ink droplet group have adjacent ink droplets at an inner side but no adjacent ink droplet at outer side, and thus only contact with the ink droplets at the inner side. By this, as shown in FIG. 3B, the ink droplets at both ends are pulled to each inner side to become respectively greater ink droplets.
The result of printing when the beading is caused has portions of higher density corresponding to the greater ink droplets at slightly inner side of the portions corresponding to the ends of the printing head. As a result, the streak at the boundary is formed together with the similar high density portion at the end of the adjacent scanning region. Such streak at the boundary due to occurrence of beading becomes more remarkable as greater amount of ink are ejected from respective printing heads for higher image density.
As can be clear from the above, beading can be easily caused when ink absorption speed of the printing medium is low. In case of the printing image recorded in the serial scanning type ink jet printing apparatus, the streak at the boundary can be caused at every printing scanning operation.
For such problem, the inventor has proposed a method to correct signals to be applied to respective printing elements in vicinity of ends of the printing head to the method set out above for obtaining the image with no streak at the boundary.
For example, it is assumed that in the printing head having the ink ejection openings arranged as shown in FIG. 4A, the density fluctuation to have higher density at both end portions as shown in FIG. 4C is caused when input signal level to be applied for ejection energy generating elements of respective ejection openings is uniform as shown in FIG. 4B. In this case, the input signal is corrected so that the input signals of a lower level are applied to the ejection energy generating elements of the ejection opening corresponding to the higher density portion as shown in FIG. 4D to reduce an ink ejection amount at the corresponding portions and whereby occurrence of beading or increasing of density at both ends due to beading can be reduced. In this case, when a printing system permits modulation of a dot diameter and/or dot density, the dot diameter to be printed by the ink ejected through the ejection opening at the ends of the head, is modulated depending upon the input signal level. For example, in a piezo type printing head, a driving voltage or pulse width to be applied to the piezoelectric element at the ends is reduced depending upon the input signal to reduce ejected ink amount for restricting increasing of dot diameter or dot density due to beading, and thus to make distribution of dot density uniform as shown in FIG. 4E. On the other hand, when modulating the dot diameter or dot density is not possible, or difficult, the number of dots to be printed is modulated depending upon the input signal to form lesser dots in the pixel corresponding to the ejection openings at the ends of the head for making density distribution uniform as shown in FIG. 4E.
By the way, as set forth above, since occurrence of the streak of density fluctuation at the boundary due to beading mainly depends on the ink absorption ability of the printing medium. Therefore, once the ink to be used for printing and the printing medium are determined, a correction amount of the image signal to be applied to the energy generation element, depends on only the image signal, namely a printing ink amount to be used corresponding to the image signal (more correctly, the printing ink amount per unit area and unit time).
On the other hand, the ink absorption speed of the printing medium may be variable depending upon moisture absorption condition in the ink absorbing surface of the printing medium due to variation of temperature and humidity of ambient air arid upon tolerance of the printing medium per production lot. In such case, it is possible that a preliminarily determined correction amount becomes inappropriate in correction for the streak at the boundary due to beading. Therefore, in a certain environmental condition, production lot of the printing medium or so forth, the streak at the boundary becomes more remarkable.